1. Field of the Invention
The present invention relates to a process for focussing a light energy beam on a reference plane of a data carrier, particularly a carrier in disk form on which the data is recorded along tracks in the form of disturbances detectable by optical means. The invention also relates to an optical device for implementing this process as well as the carrier used for this purpose.
2. Description of the Prior Art
Numerous optical systems for recording and/or reading data along tracks have been described in the prior art. When it is a question of recording data sequentially, for example video data, the tracks are generally not given material form in advance but are rather created in real time at the time of recording. The data is recorded along tracks having the form of a single spiral extending from the peripheral zone of the disk to a central zone or in the reverse direction, or else having the form of concentric circles centered on the axis of rotation of the disk.
In all these systems, arrangements must be made for ensuring correct radial tracking of the tracks during writing and/or reading, and first of all these tracks must be created in one way or another.
In the simplest systems, the accuracy of the drive means for the recording head are relied on to create these tracks at the same time as the data is written, the tracks being defined simply by the presence of these data. During reading, data written for example in the form of micro-reliefs, interfere with a reading beam focussed in the plane of the recording face of the disk. The passage of these micro-reliefs under the focussing spot modulates the beam and this modulation is detected by means of photoelectric cells converting the light intensity variations into electric signals. these electric signals may also be used for following the tracks.
The process which has just been described requires a very great mechanical stability in the advance of the recording head in order that two successive grooves do not overlap or at least so that they are not poorly discernable during reading. To improve the system which has just been described, a process has been proposed in the patent U.S. Pat. No. 4 275 275 using the last written track or one of the previously written tracks as reference.
However, when it is desired to record data in a random fashion, for example in applications relating to data processing, it is generally necessary to materialize the tracks in advance along which data may be recorded. For this, it is usual to create a pre-etching in any form whatsoever. In an embodiment described in patents U.S. Pat. Nos. 4 252 889, 4188 510 and 4 334 007, during manufacture of the carrier, the tracks are materialized in the form of a smooth groove created in an auxiliary layer of this carrier. These tracks may be detected even in the absence of any data recording, this recording being effected during a later phase in a photosensitive or heat sensitive layer in contact with the auxiliary layer.
In a preferred variation of the prior art, the pre-etched tracks may be merged with the regions in which the data is recorded. Then a so-called monotrack system is obtained.
In other processes, the pre-etched track(s) are distinct from the tracks along which the data is written, then so-called dual track or multi track systems are obtained.
The main drawback of the process which has just been described is that it does not allow maximum recording density since it requires, at least, an additional pre-etching track for a pre-etched data track. In addition, it requires the use of two beams, one for radially following the pre-etched track and the other for writing or reading data on the track intended for recording.
Carriers of the so-called mono-track type, comprising a pre-etching, are not free either of disadvantages. They generally require the use of two beams, one for recording and the other for radial tracking. Furthermore, although the pre-etched track may be readily discernable from the rest of the disk (intertrack zones) in the absence of data recording, such is not the case when data is recorded. The result may be, if precautions are not taken, inversions of contrast leading to tracking errors.
To palliate these disadvantages, a radial tracking device has been proposed using a data carrier comprising a pre-etching used for this tracking formed solely by discrete or "flag" elements spaced along the tracks.
Given effect in several variations, the pre-etching is formed by a succession of discrete non contiguous elements materializing the main axis of the tracks. The spatial distribution of these discrete elements may be uniform or not. In a first variation, each discrete element is formed by a smooth track section. In a second variation, each discrete element comprises several sections defining a particular code. In other variations, each element comprises one or more portions offset with respect to the mean axis. Finally, each of the pre-etching elements may be itself preceded by an auxiliary pre-etching element used for synchronization purposes. The track following device comprises photodetector means, measuring circuits comprising for example sampler-disablers or integrating memory circuits and sampling circuits. A radial tracking error signal is elaborated from the evolution of the signals detected during successive passages through a zone of the surface of the disk illuminated by a tracking spot.
In addition to correct tracking ensured preferably by the process which has just been described, a second requirement associated with the reading or writing of data on an optical carrier, concerns the focussing of the light energy beams used. These beams must be focussed on the carrier in a spot which must permanently follow the fluctuations in level of the tracks. For this, it is known that correct focussing of the beam is obtained by means of a focussing control device comprising a feedback loop keeping the distance between the optical device and the etched surface of the carrier constant. Locking on and maintenance of the control device in position are obtained by detecting the fluctuations of an electric signal derived, in a first variation, from reading the recorded data and, in a second variation, from detecting the pre-etching materializing the tracks, for example in the form of a smooth groove.
We then come up against the same difficulties as before recalled in connection with radial tracking.
It has also been proposed to use the same discrete pre-etching elements, in their variations comprising sections offset with respect to the mean axis of the tracks, also for ensuring focussing.
Although this process dissociates focussing from the recorded data content, accidental disturbances of the structure of the pre-etching elements play however a role in the quality of the focussing even if this role is not determining. Furthermore, the quality of the focussing is influenced by possible losses of signal due to the transitory non detection of these elements or "drop-outs", for different reasons related to the quality of the carrier and/or of the opto-electronic detection means of the pre-etching elements.